Hydroformed vehicle frame assembly and method

ABSTRACT

A method for forming a vehicle side rail structure includes forming a cylindrical tubular metal blank, forming a conical tubular metal blank, the conical tubular metal blank having a wider end portion and a narrower end portion, and joining the wider end portion of the conical tubular metal blank to one end portion of the cylindrical tubular metal blank by welding to form a welded tubular assembly. The method further includes bending the cylindrical tubular metal blank portion of the welded tubular assembly and hydroforming the welded tubular assembly by placing the welded tubular assembly into a hydroforming die cavity and pressuring an interior of the welded tubular assembly with hydroforming fluid so as to expand the welded tubular assembly into general conformity with surfaces defining the die cavity. The die cavity surfaces defines a generally octagonal shape along a portion thereof so as to conform the conical tubular metal blank portion of the welded tubular assembly therewith and thereby provide the conical tubular metal blank portion of the welded tubular assembly with a generally octagonal cross-sectional configuration.

FIELD OF THE INVENTION

[0001] The present invention is generally related to motor vehicle frame structures and more particularly to a frame assembly of tubular hydroformed construction.

BACKGROUND OF THE INVENTION

[0002] Conventional prior art motor vehicle frames are typically formed by stamping several structural components and then welding these individually stamped structures together. In more recent years, stamped and welded frame members have to some extent been replaced by hydroformed frame members.

[0003] Hydroforming is a technique that utilizes high pressure fluid to outwardly expand the metallic wall of a longitudinally extending tubular blank into conformity with the surfaces of a die cavity of a die assembly. An individual hydroformed member can have a wide range of cross-sectional geometries previously not achievable on a practical, cost-effective basis. Generally, a blank is a tubular member having a pair of open ends, a uniform cross-sectional geometry and a uniform wall thickness. During hydroforming, selected portions of the blank may be outwardly expanded to vary the cross-sectional geometry and/or circumference of portions of the blank. Outward expansion tends to reduce wall thickness in the area of the expansion which may be undesirable. Wall thinning can be compensated for to a certain degree by pushing one or both ends of the blank axially inwardly during outward expansion. This causes metal flow into the areas of outward expansion, thereby replenishing wall thickness in these areas. There are limits, however, on the amount of expansion of a single blank that can be achieved without excessive wall thinning.

[0004] When large variations in cross-sectional size and/or geometry of a hydroformed structure are required for a particular application, it has been proposed to hydroform two members separately and then join the two hydroformed members to one another after hydroforming to form a single structure having the desired geometry along its length. Individually hydroformed parts can be joined, for example, by welding. Individual hydroformed parts are highly dimensionally accurate (as compared to, for example, stamped parts). Welding two individual hydroformed parts to form a larger hydroformed structure may introduce dimensional inaccuracies in the hydroformed structure, in part because the two hydroformed members may not be properly positioned with one another prior to welding and in part because the heating and cooling of the hydroformed members that occurs during welding may distort the shape of one or both members and therefore of the resulting hydroformed structure. Specifically, the heating required to form a weld may cause unpredictable and uncontrollable distortions in the geometry of the hydroformed structure and may compromise the dimensional accuracy that would otherwise be achievable from hydroforming. Hydroforming individual parts and then welding them to one another in a subsequent manufacturing step may also increase manufacturing costs by requiring separate hydroforming dies for each member of a particular structure.

[0005] Tubular hydroforming has been proposed for the lower side rail components of vehicle ladder frame assemblies. Some vehicle frame designs require relatively large variations in cross-sectional geometry of the lower side rails, particularly to address vehicle crashworthiness. Crush caps are sometimes mounted on the forward ends of side rails and are constructed and positioned to absorb the energy of a head-on crash. It is known to hydroform a crush cap and a portion of a lower side rail separately and to join the hydroformed crush cap and hydroformed side rail at a joint in a subsequent manufacturing step. This manufacturing procedure may compromise the dimensional accuracy of the frame and may increase manufacturing costs for the reasons cited above. There is a need in the automotive industry for a more accurate and more economical method of mounting a crush cap on a vehicle side rail.

SUMMARY OF THE INVENTION

[0006] The present invention provides a method of forming a vehicle side rail structure comprising (a) forming a cylindrical tubular metal blank; (b) forming a conical tubular metal blank, the conical tubular metal blank having a wider end portion and a narrower end portion; and (c) joining the wider end portion of the conical tubular metal blank to one end portion of the cylindrical tubular metal blank by welding to form a welded tubular assembly. The welded tubular assembly is shaped by hydroforming to form the vehicle side rail structure. The welded tubular assembly may optionally be bent prior to hydroforming. For example, the cylindrical tubular metal blank portion of the welded tubular assembly may be bent. The welded tubular assembly is then hydroformed by placing the welded tubular assembly into a hydroforming die cavity and pressurizing an interior of the welded tubular assembly with a hydroforming fluid so as to expand the welded tubular assembly into general conformity with surfaces defining the die cavity. The die cavity surfaces define a generally octagonal shape along a portion thereof so as to conform the conical tubular metal blank portion of the welded tubular assembly therewith and thereby provide the conical tubular metal blank portion of the welded tubular assembly with a generally octagonal cross-sectional configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 shows a perspective view of a modular ladder frame assembly constructed according to the principles of the present invention;

[0008]FIG. 2 is a partially exploded perspective view of the frame assembly of FIG. 1 showing a forward module, a central module, and a rearward module in exploded relation to one another;

[0009]FIG. 3 is a side elevational view of a side rail assembly of the modular ladder frame assembly;

[0010]FIG. 4 is a view similar to FIG. 4 except showing portions of the side rail assembly in exploded relation;

[0011]FIG. 5 is a cross sectional view of a welded tubular assembly within a die cavity of a hydroforming die assembly shown schematically and of a pair of ram members of a hydroforming ram assembly shown schematically and in fragmentary view;

[0012]FIG. 6 is a side elevational view of the modular ladder frame assembly;

[0013]FIG. 7 is an enlarged fragmentary view of a portion of the modular ladder frame assembly as indicated in FIG. 1;

[0014]FIG. 8 is a view similar to FIG. 7 except having a portion of a connecting sleeve of the modular ladder frame assembly broken away and not shown to show more clearly a portion of a forward side rail structure of the modular ladder frame assembly; and

[0015]FIG. 9 is an enlarged fragmentary view of another embodiment of a ladder frame assembly showing a connecting sleeve thereof.

DETAILED DESCRIPTION OF THE INVENTION

[0016]FIG. 1 shows a perspective view of a ladder frame assembly 10 constructed according to the principles of the present invention. The ladder frame assembly 10 includes a forward module 12, a central module 14, and a rearward module 16. The modules 12, 14, 16 are shown in exploded relation in FIG. 2. The assembled ladder frame assembly 10 includes a pair of longitudinally extending side rail assemblies 24, 26 and a plurality of laterally extending cross structures generally designated 28 interconnected therebetween.

[0017] The side rail assemblies 24, 26 are of mirror image construction so only side rail assembly 24 will be considered in detail, but the discussion applies equally to side rail assembly 26. Corresponding portions of the side rail assemblies 24, 26 are designated by identical reference numbers to facilitate discussion of the invention. It can be understood, however, that these corresponding portions are of mirror image construction.

[0018] The side rail assembly 24 includes a forward side rail structure 30, a central side rail structure 32 and a rearward side rail structure 34. The forward module 12 includes the forward side rail structure 30 and forward cross structure interconnected therebetween. The forward cross structure includes first, second, third and fourth cross structures 36, 38, 40 and 42, respectively. The central module 14 includes the central side rail structures 32 and a central cross structure 44 rigidly interconnected therebetween. The rearward module 16 includes a pair of rearward side rail structures 34 and a rearward cross structure 46 and rearward cross assembly 48 interconnected therebetween.

[0019] The rearward side rail structure 34 includes two hydroformed rail portions 50, 52 connected at joint 54. The central side rail structure 32 is preferably a tubular structure formed by roll forming and then seam welding a sheet of metallic material. The forward side rail structure 30 is of tubular hydroformed construction and includes hydroformed rail portions 56, 58 connected at joint 60 (see FIGS. 3 and 8, for example).

[0020] The structure of the tubular hydroformed forward side rail structure 30 can be understood from FIGS. 3 and 4. Many of the structural features of the forward side rail structure 30 are formed during the hydroforming operation that creates the same. A hydroforming operation for forming the forward side rail structure 30 can be understood from FIG. 5. The forward side rail structure 30 is formed by hydroforming a welded tubular assembly 62. The welded tubular assembly 62 is comprised of a roll formed cylindrical tubular metal blank 64 and a roll formed conical tubular metal blank 66. The conical tubular metal blank 66 has a wider end portion (or wide end portion) 68 and a narrower end portion (or narrow end portion) 70. The wider end portion 68 has a larger circumference than the narrower end portion 70. The welded tubular assembly 62 is formed by welding the wide end portion 68 of the conical tubular metal blank 66 to one end portion 72 of the cylindrical tubular metal blank 64. Preferably the wide end portion 68 of the conical blank 66 is inserted inside the one end portion of the cylindrical blank 64 in telescopic relation therewith and welded to form the joint 60. Alternatively, a welded tubular assembly may be constructed by forming a conical blank that may be joined to a cylindrical blank by welding an end of the cylindrical blank inside a wide end of the conical blank. The blanks 64, 66 may be welded to one another by laser welding or any other suitable welding method. Laser welding is preferred because the joint 60 should be “fluid tight”. That is, joint 60 should be constructed to prevent a pressurized hydroforming fluid inside the welded tubular assembly from passing through the joint 60 to the outside of the welded tubular assembly during hydroforming.

[0021] Each blank 64, 66 is constructed of a suitable metallic material and has a closed transverse cross section and open tubular ends. Preferably, each blank 64, 66 is constructed of a suitable steel (such as, for example, a suitable high strength low alloy, or “HSLA”, steel) or other metallic material of sufficient strength. Each blank 64, 66 may be formed by any suitable method. For example, the cylindrical blank 64 may be formed from a continuous strip of metallic material that is shaped by roll forming and seam welded to have a closed transverse cross section. The continuous tubular structure may then be cut to the length required. The conical tubular metal blank may be formed by roll forming and seam welding a wedge-shaped sheet of metallic material.

[0022] The cylindrical portion of the welded tubular assembly 62 is preferably bent prior to being placed in a hydroforming assembly. As shown in FIG. 5, for example, the cylindrical blank 64 of the welded tubular assembly 62 is bent in two places. If a relatively “sharp” bend or angle (that is, an angle greater than 30 degrees) is required in the tubular hydroformed structure, the present invention may bend and hydroform the blank 64 according to the teachings of U.S. Pat. No. 5,953,945 entitled METHOD AND APPARATUS FOR WRINKLE-FREE HYDROFORMING OF ANGLED TUBULAR PARTS, which patent is hereby incorporated by reference into the present application in its entirety. The blank 64 portion of the welded tubular assembly 62 may be bent in a computer numeric controlled (“CNC”) bending machine prior to being placed in the die assembly or, alternatively, may be bent by stretch bending, or by any other suitable method. A suitable lubricant may be applied to the exterior of the welded tubular assembly 62 prior to placing it in a die assembly.

[0023] The welded tubular assembly 62 is then placed between the die halves 74, 76 of the die assembly 78 and the assembly is closed. The welded tubular assembly 62 is preferably immersed in a fluid bath so that it is filled with hydroforming fluid (not shown). A hydroforming ram assembly 80, 82 (a portion of each is represented schematically in FIG. 5) is engaged with each end 70, 84 of the welded tubular assembly 62 such that a ram member 86, 88 of each assembly 80, 82 seals an end 84, 70, respectively, of the welded tubular assembly 62. The ram members 86, 88 include hydraulic intensifiers which can intensify the hydroforming fluid, thereby increasing the fluid pressure of the fluid within the welded tubular assembly 62 to outwardly expand the tubular metallic walls, 90, 92, respectively, of the tubular blanks 64, 66 of the assembly 62 into conformity with the die surfaces 94 of the die cavity to thereby form a tubular hydroformed side rail structure 30 having an exterior surface that is fix into a predetermined (determined by the shape of the die cavity) configuration. Preferably each blank 64, 66 is constructed of the same metallic material and the walls 90, 92 are of approximately equal thickness to one another, although neither of these conditions is required by the invention. That is, the blanks 64, 66 may be constructed of different materials from one another and/or the thickness of the walls 90, 92 may be different from one another.

[0024] The cylindrical blank 64 may have, for example, an essentially constant circumference and an essentially circular cross section along its length prior to outward expansion during the hydroforming operation. The conical tubular blank 64 may have, for example, an essentially circular cross section at each point along its length. Thus, the circumference of the conical blank 66 decreases from the wide end 68 to the narrow end 70 (prior to outwardly expansion), but the cross section at any point is circular. The hydroforming process may be computer controlled to control the flow of the hydroforming fluid to thereby control the manner in which the metallic material of the welded tubular assembly 62 expands (in a radial direction) during the hydroforming process. The die cavity of the die assembly 78 is shaped to provide a rearward portion of the welded tubular assembly 62 with a quadrilateral (preferably rectangular) cross-section, the forward portion of the assembly 62 with an octagonal cross section, and an intermediate transition portion of the welded tubular assembly 62 with a cylindrical cross section in the area of the welded joint 60 therebetween. This construction is best appreciated from FIG. 8.

[0025] The fluid pressure and the axial pressure may be controlled independently of one another. The ram members 86, 88 may push axially inwardly on opposite ends 70, 84 of the welded tubular assembly 62 to create metal flow within the walls 90, 92 of the assembly 62 during outward expansion to maintain the thickness of each wall 90, 92 within a predetermined range of its original wall thickness. The ram members 86, 88 may be operated, for example, to maintain the wall thickness of each wall 90, 92 of the respective tubular blanks 64, 66 within about +/−10% of its original wall thickness.

[0026] The fluid pressure within the welded tubular assembly 62 causes the walls 90, 92 to expand into conformity with the surfaces 94 defining the hydroforming die cavity so as to irregularly outwardly expand the walls into conformity with the surfaces 94 and thereby provide the welded tubular assembly 62 with a shape corresponding to the forward side rail structure 30. The shape of the die cavity thus corresponds to the shape of the forward side rail structure 30. More particularly, the die cavity defines a generally octagonal shape along a portion thereof so as to conform the conical tubular metal blank portion 66 of the welded tubular assembly 62 therewith and thereby provide the conical tubular metal blank portion 66 of the welded tubular assembly 62 with a generally octagonal cross-sectional configuration as a result of the hydroforming operation.

[0027] If holes are to be formed in a hydroformed forward side rail structure 30, the holes may be formed while the side rail structure 30 is in the die assembly 78 during the hydroforming operation or may be formed after the structure 30 is removed from the die assembly along with any other required further processing of the structure 30. More particularly, holes may be formed during the hydroforming process in what is known as a hydropiercing operation. A hydropiercing operation is disclosed in U.S. Pat. No. 5,460,026 which is hereby incorporated by reference in its entirety into the present application. Alternatively, holes or notches of various sizes and shapes may be cut (preferably using a laser) in the forward side rail structure 30 after the hydroforming operation is completed.

[0028] It can be appreciated that, as a result of the expansion of the welded tubular assembly 62 during the hydroforming operation, the transverse cross section of the forward side rail structure 30 varies along its length to have quadrilateral (preferably rectangular), octagonal, and cylindrical cross sections as described above. It is also contemplated to hydroform the various portions of the forward side rail structure 30 to have other cross sectional configurations (including other sizes and shapes). It can thus be understood that altering the cross-sectional configuration of the tubular hydroformed forward side rail structure 30 can be accomplished without departing from the principles of the present invention.

[0029] It can be appreciated that the rearward side rail structure 34 may be formed by carrying out a method similar to the method for making the forward side rail structure 30. That is, two or more tubular blanks (each of which may be formed by roll forming) may be welded together to form a welded tubular assembly. The welded tubular assembly may be bent and hydroformed to form the rearward side rail structure 34.

[0030] The ladder frame assembly 10 may be constructed by first assembling the modules 12, 14, 16 and then connecting the assembled forward and rearward modules 12, 16 to the forward and rearward ends, respectively, of the assembled central module 14. The assembled modules 12, 14, 16 are shown in FIG. 2.

[0031] To assemble the rearward module 16, the rearward cross assembly 48 and the rearward cross structure 46 may be interconnected between the rearward side rail structures 34 by welding or other appropriate method. To assemble the central module 14, the central cross structure 44 may be connected between the central side rail structures 32 by welding or other appropriate method. The forward modules 12 may be assembled by welding the cross structures 36, 38, 40, 42 between the forward side rail structures 30. Each cross structure 36, 38, 40, 42 is preferably a metallic structure that is constructed of one or more pieces that have each been shaped by stamping and welded together. The cross structures 38, 40 and 42 are connected between the quadrilateral rearward portions of the forward side rail structure 30. The manner in which the cross structure 36 is connected between the forward side rail structure 30 can be appreciated from FIGS. 7 and 8.

[0032] A pair of connecting sleeves 104, 106 are disposed in surrounding relation to respective forward side rail structures 30. The connecting sleeves 104, 106 are identical so only sleeve 104 will be discussed in detail, but the discussion applies equally to sleeve 106. The connecting sleeve 104 has a relatively narrow end 108 and a relatively wide end 110. The connecting sleeve 104 is disposed on the forward side rail structure 30 in the vicinity of the welded joint 60 of the hydroformed welded tubular assembly 62. Specifically, the relatively narrow end 108 of the sleeve 104 is welded in the vicinity of the joint 60 to the exterior of the forward end of the portion of the side rail structure 30 formed from the cylindrical tubular blank 64.

[0033] The relatively wide end 110 of the connecting sleeve 104 provides an attachment surface to which one end of the first cross structure 36 may be attached. Opposite ends of the first cross structure 36 are thus connected to respective connecting sleeves 104, 106 so that the cross structure extends between the sleeves 104, 106. The connecting sleeve 104 thus provides a support structure and an attachment surface on the forward end of the side rail assembly 24 for the first cross structure 36. As best appreciated from FIG. 7, the wide end portion 110 of the connecting sleeve 104 is spaced radially outwardly from the relatively smaller diameter octagonally shaped forward end of the forward side rail structure 30. The octagonal forward end portion of the side rail structure 30 is constructed and arranged to be “crumpled” or crushed in the event that the front end of a vehicle constructed using the ladder frame assembly 10 is involved in a head-on vehicle accident. The connecting sleeve 104 is constructed to allow crumpling of the octagonal forward end portion (that is, the portion forward of the joint 60) of the forward side rail structure 30 while providing a secure attachment structure for the first cross structure 36 to maintain the structural integrity of the vehicle frame 10 during crumpling of the octagonal end. For example, the wide end 110 of the connecting sleeve 104 provides an interior space constructed and arranged to allow crumpling of the forward end portion of the side rail structure 30 during a collision without breaking or weakening the joint between the first cross structure 34 and the sleeves 104, 106. The sleeve may be of one-piece construction and may be made of a metallic material that has been shaped by roll forming and seam welding.

[0034] The connecting sleeve 104 is partially broken away and not shown in FIG. 8 to show the structural details of a portion of the forward module 12 in the vicinity of the sleeve 104 and of the joint 60 more clearly. The construction of the joint 60 (formed between the tubular blanks 64, 66 prior to hydroforming) can be understood from FIG. 8. Specifically, a series of circumferentially spaced openings 112 are formed in the end 72 of the blank 64. The conical blank 66 is inserted into the end 72 of the cylindrical blank 64 and the joint 60 is formed by welding. The blanks may be welded together using laser welding, as mentioned above. Welding material may be disposed within the openings 112 to secure the two blanks 64, 66 together. It can also be appreciated from FIG. 8 that the relatively narrow end 108 of the connecting sleeve 104 is welded (by mig welding, for example) to a cylindrical exterior surface portion of the forward side rail structures 30 in the vicinity of the joint 60 and that the relatively wide end 110 of the sleeve 104 extends forwardly from the joint 60 and radially outwardly spaced from a portion of the octagonal forward end portion of the side rail structure 30.

[0035] The hydroformed construction of the forward side rail structure 30 can also be understood from FIG. 8. Specifically, it can be understood that the forward side rail structure 30 is hydroformed to have a relatively large circumference rearward end portion 120 having a quadrilateral (preferably rectangular) cross-section, a relatively small circumference forward end portion 122 having an essentially octagonal cross section and an intermediate (or “transition”) portion 124 therebetween in the vicinity of the joint 60. The intermediate portion 124 has an essentially circular cross-section. The octagonal forward end portion 122 is tapered so that its circumference continuously decreases in a direction forwardly of the joint 60 and terminates in an octagonal tubular open end 126. Four circumferentially spaced indentations or depressions 128 are formed near the open end 126 of the forward end portion 122 of the side rail structure 30 during hydroforming. The depressions 128 are constructed and arranged to facilitate crush initiation during a collision.

[0036] A more preferred embodiment of the connecting sleeve is shown in FIG. 9. FIG. 9 shows a connecting sleeve 130 mounted on the forward side rail structure 30 of a vehicle ladder frame assembly 132 (shown in fragmentary view). Portions of the vehicle ladder frame assembly 132 that are identical to portions of the frame 10 are identified by identical reference numerals and are not described further. The connecting sleeve 130 is of multi-piece construction. Preferably, two sleeve members 133, 135, each having a C-shaped cross section are welded together to form the sleeve 130. Each member 133, 135 is preferably constructed of a metallic material that has been shaped by stamping. Each member 133, 135 includes a wall structure 134, 136, respectively, which is preferably constructed of a strip of metallic material having opposite free end portions. The members 133, 135 are disposed in surrounding relation around the side rail structure 30 in the vicinity of the joint 60 of the forward side rail structure. Opposite ends of the wall structure 136 (of member 135) are “lapped” (that is, disposed in overlying relation) over and welded to associated ends of the wall structure 134 (of the member 133) to form the sleeve 130. The sleeve 130 is disposed around the forward side rail structure 30 in the vicinity of the joint 60. Preferably the sleeve 130 is welded to the side rail structure 30 by mig welding, although any appropriate welding method can be used. The wall structures 134, 136 are shaped to provide the sleeve 130 with a relatively narrow end 138 which is attached to the side rail structure 30 in the vicinity of the joint 60 and a relatively wide end 140. The relatively wide end 140 provides attachment structure for and an attachment surface for the first cross structure 36. One or more weld openings 142 are provided in the portions of the wall structures 134, 136 which define the relatively narrow portion 138 of the sleeve 130. The welding material (not shown) may be disposed in each opening 142 (preferably by mig welding) to help secure the sleeve 130 to the forward side rail structure 30.

[0037] Alternatively, a connecting sleeve (similar to sleeve 130, for example) may be of one-piece construction. When this construction is employed, a single strip of a metallic material may be wrapped in surrounding relation around the front side rail structure 30 in the vicinity of the joint 60, clamped thereto, and welded to form the connecting sleeve.

[0038] It should be pointed out that although the ladder frame assemblies described herein are referred to as “modular”, this characterization is intended to be broadly construed and is not intended to limit the manner in which any of the ladder frame assemblies is constructed. It is contemplated that each module (such as modules 12, 14, 16 of the ladder frame assembly 10) be assembled separately and then assembled to one another to form the ladder frame assembly. An example of a method for connecting the forward and rearward modules 12, 16 to the central module 14 can be understood from FIG. 1. Each end of each roll formed central side rail structure 32 may be provided with a “fish mouth” opening. The rearward fish mouth openings 150 of the central module 14 are constructed and arranged to receive the forward end portion of each rearward side rail structure 34 of the rearward module 16 therein in telescopic relation. Similarly, the forward fish mouth openings 152 are constructed and arranged to receive the rearward end portions of the forward side rail structures 30 of the forward module 12 therein in telescopic relation. Each fish mouth opening 150, 152 may then the crimped to hold the associated side rail structure 34, 30 therein and each forward and rearward side rail structure 30, 34 may then be welded (for example by mig welding) in the associated fish mouth opening 152, 150.

[0039] It is contemplated to construct each ladder frame assembly in a variety of ways, however, and so it is to be understood that no limitations on the order in which the various hydroformed members and other structural members are joined together to form each ladder frame assembly is to be implied from anything shown or stated in the present application. For example, the ladder frame assembly 10 may be constructed by first connecting the forward side rail structures 30 and the rearward side rail structures 34 to respective ends of the central side rail structures 30 to as illustrated in FIGS. 3 and 4 to form the pair of assembled side rail assemblies 24, 26, and then connecting the assembled side rail assemblies 24, 26 in laterally spaced relation to one another by connecting the cross structure 28 therebetween.

[0040] Thus, it can be appreciated that although the ladder frame assembly 10 in FIG. 2 is show in exploded view as a series of assembled modules 12, 14, 16, it is understood that, while it is contemplated and preferred to completely assemble each module separately before the modules are connected together to form the ladder frame assembly, this is not required by the invention and the invention is therefore not limited to this method of construction.

[0041] It can be understood that the modular approach allows a particular module to be used in the construction of a wide range of ladder frame assemblies. For example, the central module 14 of the ladder frame assembly 10 shown in FIG. 1 generally defines the passenger compartment portion of a vehicle. Several different forward and rearward modules can be constructed for use on a central module having a particular construction to provide ladder frame assemblies having different configurations and/or different lengths. A range of forward modules can be easily constructed, for example, to accommodate a wide range of vehicle front configurations. Similarly, the rear module can be reconfigured to provide different ladder frame assembly lengths (and thus different vehicle lengths) and a variety of vehicle styles and appearances. It is contemplated to provide a wide range of forward modules that include forward side rail structures each being constructed from a welded tubular assembly that has been hydroformed to define a tapered forward end portion having an octagonal cross-section as described above.

[0042] Thus, while the invention has been disclosed and described with reference with a limited number of embodiments, it will be apparent that variations and modifications may be made thereto without departure from the spirit and scope of the invention. Therefore, the following claims are intended to cover all such modifications, variations, and equivalents thereof in accordance with the principles and advantages noted herein. 

In the claims:
 1. A method for forming a vehicle side rail structure, comprising: forming a cylindrical tubular metal blank; forming a conical tubular metal blank, said conical tubular metal blank having a wider end portion and a narrower end portion; joining said wider end portion of said conical tubular metal blank to one end portion of said cylindrical tubular metal blank by welding to form a welded tubular assembly; bending said cylindrical tubular metal blank portion of said welded tubular assembly; hydroforming said welded tubular assembly by placing said welded tubular assembly into a hydroforming die cavity and pressuring an interior of said welded tubular assembly with hydroforming fluid so as to expand said welded tubular assembly into general conformity with surfaces defining said die cavity, said die cavity surfaces defining a generally octagonal shape along a portion thereof so as to conform said conical tubular metal blank portion of said welded tubular assembly therewith and thereby provide said conical tubular metal blank portion of said welded tubular assembly with a generally octagonal cross-sectional configuration.
 2. A method of forming a vehicle frame module including a pair of side rail structures interconnected by a cross structure, comprising: (i) forming each of said pair of side-rail structures by: forming a conical tubular metal blank, said conical tubular metal blank having a wider end portion an a narrower end portion; joining said wider end portion of said conical tubular metal blank to one end portion of said cylindrical tubular metal blank by welding to form a welded tubular assembly; hydroforming said welded tubular assembly by placing said welded tubular assembly into a hydroforming die cavity and pressuring an interior of said welded tubular assembly with hydroforming fluid so as to expand said welded tubular assembly into general conformity with surfaces defining said die cavity, said die cavity surfaces defining a generally octagonal shape along a portion thereof so as to conform said conical tubular metal blank portion of said welded tubular assembly therewith and thereby provide said conical tubular metal blank portion of said welded tubular assembly with a generally octagonal cross-sectional configuration; (ii) forming a pair of connecting sleeves, each formed separately from said side rail structures, said connecting sleeves having a relatively narrowed end and a relatively wider end; (iii) disposing said connecting sleeves in surrounding relation to respective ones of said side rail structures in the vicinity of said welded joints of said hydroformed welded tubular assemblies; (iv) welding said relatively narrow ends of said connecting sleeves to said respective ones of said side rail structures; (v) attaching opposite ends of a cross member to said connecting sleeves so that said cross member extends between said connecting sleeves.
 3. A method according to claim 2, further comprising bending said cylindrical tubular metal blank prior to said hydroforming.
 4. A method according to claim 2, wherein said connecting sleeves are each formed from a pair of sleeve members each constructed of a metal material and each sleeve member having two free end portions and wherein said disposing comprises placing each pair of members in surrounding relation with one of said side rail structures in the vicinity of the welded joint thereof and welding associated free end portions of each pair of sleeve members to one another.
 5. A method according to claim 4, wherein each sleeve member of each said pair has a C-shaped cross section.
 6. A method according to claim 4, wherein said relatively narrow ends of said connecting sleeves are each welded to a portion of an associated side rail structure that was formed from said cylindrical tubular metal blank portion. 